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In this study, nine strains of Aedes aegypti mosquitoes from different localities in Thailand were subjected to susceptibility tests against commonly used insecticides for vector control in Thailand. Aedes aegypti from different localities were strongly resistant to DDT and permethrin, except for one strain from Chiang Mai (Chiang Mai-P) that was found to be susceptible to permethrin. In contrast, most Ae. aegypti strains are susceptible to deltamethrin and alphacypermethrin, except that Bangkok and Nonthaburi strains showed incipient resistance to deltamethrin and alphacypermethrin, respectively. Various levels of malathion and propoxur susceptibility were also detected in Ae. aegypti strains; however, two strains of Ae. aegypti (Satun and Nonthaburi) were found to be completely susceptible to malathion. The insecticide susceptibility level in three strains of Culex quinquefasciatus was also investigated. The results indicated strong resistance to DDT and moderate resistance to permethrin. Two strains from Bangkok and Nonthaburi were found to be completely susceptible to malathion and propoxur. One strain from Pathum Thani showed incipient resistance to both compounds. We conclude that deltamethrin and alphacypermethrin are still effective insecticides for the dengue control program and malathion and propoxur may be alternative insecticides to control Cx. quinquefasciatus.